Sensing of ammonia at room temperature by polypyrrole-tin oxide nanostructures: Investigation by Kelvin probe force microscopy

• The nanowires of pure polypyrrole (PPy) and polypyrrole-tinoxide nano-composites have been prepared by chemical oxidation method.
• The sensitivity of the composite sample is greater than the pure PPy and SnO2 samples at room temperature.
• Kelvin probe force microscopy (KPFM) system is used for imaging the potential changes on the surface of the sample.
• Faster and larger sensing response of PPy-SnO2 composite towards ammonia have been attributed to the formation of p-n junctions at the interface between PPy nanowires and SnO2 nanoparticles.

The present investigations focus on gas sensing response of nanowires of pure polypyrrole (PPy) and polypyrrole-tinoxide nano-composites (PPy-SnO2). These composites comprise of polypyrrole nanotubes enveloped around tin oxide nanoparticles. The sensing response of these nanowires towards ammonia gas has been estimated by measuring changes in the surface resistance of the samples in ammonia with respect to air. At room temperature, at 100 ppm of ammonia, the sensitivity of pure PPy nanowires is about 18%, whereas no response has been observed in the SnO2 particles upto 250 °C. However, it is intriguing that the sensitivity (∼26%) of the composite sample is greater than the pure PPy sample at room temperature. Also, the sensitivity increases with an increase in ppm level of ammonia. To probe this sensing behavior, we have employed Kelvin probe force microscopy (KPFM) system for imaging the potential changes on the surface of the sample. Ammonia gas was introduced in a specially designed cell. With the help of our KPFM investigations, it has been proposed that the faster and greater response of sample PPy-SnO2 towards ammonia can be attributed to the presence of large number of charge compensating sites/diode interfaces which are absent in pure PPy sample.